Water management is routinely listed as one of the top global challenges facing mine development, production, and closure. Successful mine water management can mean the difference between operating at a profit or loss. This approach benefits the mine operations by lowering costs and effectively managing water quantity and quality while controlling adverse hydrological impacts. Moreover, government environmental agencies throughout the world are continuously moving to impose progressively more challenging minimum standards for water quality in mine water effluent discharge, in view of protecting fish and fresh water plants, as well as humans (drinking water).
The principal waste-waters associated with mines and quarries are slurries of rock particles in water. These arise from rainfall washing exposed surfaces and haul roads and also from rock washing and grading processes, and also from seepage. Volumes of water can be very high, especially rainfall related arising on large sites. Some specialized separation operations, such as coal washing to separate coal from native rock using density gradients, can produce wastewater contaminated by fine particulate haematite and surfactants. Oils and hydraulic oils are also common contaminants. Wastewater from metal mines and ore recovery plants are inevitably contaminated by the minerals present in the native rock formations. Following crushing and extraction of the desirable materials, undesirable materials may become contaminated in the wastewater. For metal mines, this can include unwanted metals such as zinc and other materials such as arsenic. Extraction of high value metals such as gold and silver may generate slimes containing very fine particles where physical removal of contaminants becomes particularly difficult.
Arsenic is a chemical element (symbol As), atomic number 33 and relative atomic mass 74.92. Arsenic is a naturally occurring contaminant found in many ground waters. It generally occurs in two forms (valence or oxidation states): pentavalent arsenic (arsenate) and trivalent arsenic (arsenite). In natural ground water, arsenic may exist as trivalent arsenic, pentavalent arsenic, or a combination of both. Although both forms of arsenic are potentially harmful to human health, trivalent arsenic is considered more harmful than pentavalent arsenic. Moreover, trivalent arsenic is generally more difficult technically to remove from drinking water than pentavalent arsenic. That is why an efficient method for arsenite removal from contaminated water solution is still not available.
Arsenate is a salt or ester of arsenic acid. Natural arsenite minerals are very rare oxygen bearing arsenic minerals, but may be synthesized in industrial plants.
Arsenic is a metalloid. The main use of metallic arsenic is for strengthening alloys of copper and especially lead (for example, in car batteries). Arsenic is also common in semi-conductor electronic devices and in the production of pesticides, herbicides and insecticides. Arsenic is highly toxic and very poisonous to multicellular life, including humans, by blocking the Krebs cycle essential to cellular metabolism which results in loss of ATP. In particular, arsenic contamination of groundwater is a public health issue that affects millions of people across the world.
In water contaminated streams in the wild, both arsenite and arsenate forms of arsenic will be found, dynamically interacting with each other, with relative concentration balance of one or the other shifting according to various parameters including electromagnetic radiations (e.g. ultraviolet sun radiations) and biological elements (e.g. bacteria). Some bacteria obtain their energy by oxidizing various fuels while reducing arsenates to form arsenites, while other bacteria will oxidize arsenites to form arsenates via photosynthesis.
A one-step process involving lime can be used for purification of a water source contaminated with arsenic. However, this water purification process does not meet newer arsenic discharge limits.
It is possible to remove arsenic by oxidation with the addition of an oxidizing agent such as for example potassium permanganate, molecular oxygen, chlorine, calcium hypochlorite and ferric iron. These processes can use either one or two steps and will reduce arsenic concentrations, but are costly to operate because of the oxidation requirements, and may be hazardous to mammals with some of these components being recognized as carcinogenic compounds, and thus undesirable for potable water treatment. Such oxidizer based water purification methods will also require relatively high liquid water solution temperatures, for example between 40° and 70° Celsius, and will become inoperative in cold and particularly in very cold temperatures found in cold stream waters for example in Canada and Chile. High operating temperatures will have a detrimental environmental impact over and above the economic cost, since water to be treated will have to be previously warmed up.
Moreover, if ammonia is present in the raw (untreated) water solution, as is often the case with industrial effluents, oxidizers may generate toxic by-products such as trichloromethane (chloroform) and chloramines, which are toxic to mammals.
Also, it is possible, after oxidation, to absorb arsenic on solid media such as for example clay, granular ferric hydroxide, colloids, alumina and activated silica. However, such absorption will require a lot of absorption media and will generate a large amount of toxic solid wastes, wherein their disposal will become a major economic drain in large scale industrial plants, particularly for arsenic. The media life will be too short.
In contaminated water solutions, it is possible for pentavalent arsenic (arsenate) to be removed by coagulation with ferric chloride and with a softening process using lime. However, such coagulation does not remove trivalent arsenic (arsenite) as efficiently as in the case of arsenate.